This invention relates to a pressure developing device for developing with a pressure force applied to a recording medium, more particularly, to a pressure developing device having a function for detecting width of the recording medium to pressurized and being capable of varying pressure force in accordance with the detected width of the recording medium.
Sensitized paper of microcapsule type for use with color duplicators and color image recording devices is disclosed as already known in Japanese Patent Provisional Publication SHO58-23025 and SHO58-88739. There are two different types of such sensitized paper, self-developing type and transfer type.
Self-developing type sensitized paper consists of a layer of developer material coated on plain paper, which layer is further coated with a layer of mircocapsules containing dye precursor, photo-hardening resin, sensitizer and polymerization initiator. When light from a light source shines such sensitized paper thorough a text for instance in the form of photo-film, the microcapsules are photo-harden only on the area correpsonding to the text area through which the light are passed. The sensitized paper is then passed between the pressure rollers of the pressure developing device to apply pressure force to the paper for developing a visible image. This causes the microcapsules remaining unharden to be crushed on the area corresponding to another text area that has allowed no light beams to pass. The precursor etc. contained in the crushed microcapsules then undergoes color-developing reaction with the developer material on the developer layer to thereby effect color development.
In case of the transfer type sensitized paper, on the other hand, the sensitized paper carries on its surface a microcapsule layer alone with no coating of developer material, separately requiring sheets of developing paper with a developer layer only. When using such transfer type paper, after certain exposure treatment of the sensitized paper, the developing paper is laid upon the exposed surface of the sensitized paper. The two sheets of paper are then pressurized together by the pressure developing device to cause a color developing reaction on the developing paper, so that a color image appears on the developing paper.
With such sensitized paper pressure-developed by the pressure developing device as described above, a high-quality picture that has not been available on conventional toner images is obtained in either self-developing type or transfer type.
For such a pressure developing device, an arrangement with a pair of pressure rollers is widely used because of its advantages in space and cost. In conventional pressure developing devices, sizes of sensitized paper (in particular, widths) in major use are preset so that a certain pressure is applied to the paper over the preset size of the paper extending longitudinally of the pressure roller pair.
When developing the self-developing sensitized paper by means of a pressure developing device, however, a pressure force applied between the pressure rollers must be in the order of 90 kg/cm to crush the unharden microcapsules on the sensitized paper, in consideration of the weight resistance of the sensitized paper and other factors. Taking for example a conventional pressure developing device in which a pair of pressure rollers are so pressurized that this amount of pressure force should be applied to the sensitized paper of a preset size (A4: 210 mm.times.300 mm), if a sheet of paper smaller than the preset paper size (A4) is placed between the pressure rollers, the pressure force applied to the sensitized paper is higher than the pressure force applied to the paper (about 90 kg/cm) mentioned above. If oppositely the paper larger than the preset size (A4) is placed between the pressure rollers, the pressure force applied to the sensitized paper is lower than the pressure force applied to the paper (about 90 kg/cm). For this reason, in conventional pressure developing devices, when any piece of paper whose size is different from the preset size is placed between the pressure rollers, color strength is most likely different among paper sizes because the pressure force for each paper size is different, resulting in an uneven picture quality.
Further in conventional pressure developing devices, the pressure force to the both ends of the paper width has been substantially the same as that applied to the middle part of paper. This has caused such problems that, since sheets of sensitized paper is very thin in general, paper is apt to wrinkle when it is inserted between the pressure rollers.